Research on molecular beam epitaxial growth of gallium selenide thin films
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Broadening the variety of two-dimensional (2D) semiconductors is crucial to the semiconductor development roadmap since their van der Waals (vdW) properties benefit the fabrication of heterostructures with multiple functions. Layered chalcogenides have garnered significant attention as emerging 2D materials, owing to their diversity and versatile properties. GaSe is a prominent member of this family, valued for its potential in optics, electronics, and optoelectronics. GaSe is well-suited for compact heterostructure devices due to its facile fabrication into atomic-scale ultrathin films. It also exhibits remarkable properties, including a bandgap transition from an indirect 3.3 eV in a single layer to a direct 2.1 eV in bulk, p-type conductivity, nonlinear optical behaviors, and high transparency across 650-18000 nm. These make GaSe a promising material for transistors, photodetectors, and photovoltaics. However, challenges persist in achieving wafer-scale synthesis. ☐ This study investigated the molecular beam epitaxy (MBE) synthesis of GaSe and achieved high-quality wafer-scale GaSe thin films. We explored GaSe growth on c-plane sapphire and GaAs (111)B substrates and examined the impact of growth parameters, including substrate temperature, flux ratio, and growth rate. Structural and optical properties of GaSe thin films were characterized. First-principles calculations were employed to analyze the GaSe growth and GaAs (111)B wafer processing mechanisms. Machine learning was used to build Bayesian interface models for guiding and predicting MBE synthesis experiments. ☐ We obtained GaSe single-crystal films (about 32 nm thick) on c-sapphire with a root mean square (RMS) roughness of 1.82 nm using optimized growth parameters. We further developed a three-step mode to fabricate 3-layer GaSe films with enhanced crystallinity and surface morphology, achieving an RMS roughness of 0.61 nm. On GaAs (111)B substrates, we systematically explored the growth window for GaSe and observed the gamma'-GaSe polymorph for the first time. We also demonstrated the respective advantages and limitations of 2D substrates (e.g., sapphire) and 3D substrates (e.g., GaAs) for GaSe growth, revealing distinct mechanisms of vdW and quasi-vdW epitaxy. Using experimental databases, we developed machine learning models to predict the crystallinity and surface morphology of GaSe films based on input growth parameters. In addition, we provided comprehensive insights into GaAs (111)B wafers, including deoxidation, passivation, and preservation. ☐ This study advances the wafer-scale production of high-quality GaSe single-crystal thin films. The discovery of gamma'-GaSe, with its centrosymmetric unit layer structure, opens avenues for exploring unique properties such as enhanced optoelectronic performance. Furthermore, our work provides valuable insights into the MBE growth of both 2D/2D and hybrid 2D/3D heterostructures, broadening material potential for device applications and establishing a foundation for integrated quantum photonic devices. Ongoing research aims to further develop GaSe as a platform for quantum technologies and extend the machine learning-based automated MBE synthesis platform to more vdW chalcogenide materials.
Description
Keywords
Automated synthesis platforms, Heterostructures, Machine learning, Molecular beam epitaxy, Semiconductor thin films